1. Field of the Invention
This invention concerns glass molding techniques and especially techniques for obtaining tempered shaped glass sheet, used in automobiles or other vehicles forwindshields, side windows, rear windows or even sliding roofs.
2. Background of the Prior Art
The manufacture of such automobile windows is usually obtained by a sequence of three different stages, heating of the glass to its softening point, deformation of the flat glass sheet to make it adopt the desired curvature and, finally, a tempering of the glass to give it greater shock resistance and greater splintering in case of breakage. One of the industrial methods most often used consists of heating of the glass in a horizontal heating furnace in which the flat glass sheets are loaded one by one, and conveyed, for example, on a roller bed. The conveyor is extended beyond the heating furnace and then carries the glass sheet to the molding station. The glass sheet is bent into convex shape there and is then released before being sent to the tempering station.
Patent publication EP No. 0 003 391, in the names of Messrs. Harold McMaster, Norman Carl Nitschke and John Stephen Nitschke, describes a system specially adapted for the manufacture of tempered automobile windows, which contains a horizontal furnace crossed by a conveyor, a bending system and a tempering system. Bending is carried out on a frame where the glass sheet sinks under the effect of inertia and gravity and takes the desired shape before being conducted to the cooling or tempering station.
Transfer from the conveyor crossing the furnace to the bending frame is obtained by suction of the glass sheet, which is flattened against a flat upper suction mold of refractory mortar, drilled with multiple holes over all or part of its surface and greater in area than that of the glass sheet to be molded. Those holes, regularly distributed, connect with a vacuum chamber, so that the whole surface of the glass sheet is sucked up to the upper mold. The bending frame is then brought under the lifted sheet and collects it after the vacuum has ceased.
Molding and tempering systems of this type are widely used owing to the rapid production rates they make it possible to obtain, while respecting the optical qualities required by automobile manufacturers.
However, such systems are specially reserved for mass production and are generally poorly suited for the treatent of series of windows presenting relatively appreciable differences from each other in dimensions, on the order, for example, of at least a few centimeters.
Actually, with an upper mold, such as described in patent No. EP 0 003 391, it is certainly possible to take in succession volumes of glass of different sizes, but, in practice, it was observed that good optical results could be assured only when the volumes of a new series did not cover the zone occupied by the edges of the volumes previously treated. Furthermore, any change of the upper mold raises numerous problems of handling the heavy refractory mortar mold and of setting temperature.
Additionally, difficult problems are created by enameled windows, that is, those bearing an enamel coating over all or part of one of their faces. Now, they constitute at present the great majority of windows, enamel having, among other things, an esthetic function in the case of enamel strips deposited along the edges of windows mounted by cementing, particularly in automobiles; furthermore, enamel prevents the cement from deteriorating under the sunlight, a sun visor function in the case of sliding roofs or also a function of electricity conduction and/or of covering electric leads or "busbars" for rear windows of vehicles equipped with self-deicing systems. This unrestricted enumeration shows how uncommon automobile windows not coated with enamel are.
The enameling techniques used demand vitrification at a temperature equivalent to that necessary for bending and/or tempering of the glass. It has, of course, been proposed that this vitrification be carried out in the heating furnace with a view to bending and tempering, which means that the glass sheet is loaded in said furnace, while it presents a nonvitrified, generally cloudy appearance on the top face, that is, on the face which is not in contact with the conveyor and will, consequently, later be in contact with the upper mold, while the enamel in the course of vitrification is still very fragile, sensitive to abrasion and sticky.
That contact between the enamel and the upper mold entails, on the one hand, a rather appreciable deterioration of the enameled strips and, on the other, soiling of the upper mold. Furthermore, the contact surface of the upper mold is very often covered with refractory paper or cloth, which produces a softer contact with the glass, but on which the enamel has a strong tendency to stick, which necessitates frequent replacements of the refractory paper or cloth and, therefore, multiple production interruptions. To reduce their frequency, one can, of course, choose to treat volumes of decreasing sizes, thus preventing unenameled glass surfaces from coming in contact with the mold on the site of the enamel strips of a previously treated volume. But, on the one hand, that precaution requires particular care in the planning of bending-tempering treatment and, on the other, yields only a limited solution, for a frequent change of refractory paper or cloth coaing remains necessary. Furthermore, according to the technique of patent EP No. 0 003 391, each new paper or cloth coating must be bored according to a specific pattern to enable the vacuum through the holes of the refractory mortar mold to be exerted.